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• Storing a jpa entity where only the timestamp changes results in updates rather than inserts (desire

  - by David Schlenk

  I have a JPA entity that stores a fk id, a boolean and a timestamp: @Entity public class ChannelInUse implements Serializable { @Id @GeneratedValue private Long id; @ManyToOne @JoinColumn(nullable = false) private Channel channel; private boolean inUse = false; @Temporal(TemporalType.TIMESTAMP) private Date inUseAt = new Date(); ... } I want every new instance of this entity to result in a new row in the table. For whatever reason no matter what I do it always results in the row getting updated with a new timestamp value rather than creating a new row. Even tried to just use a native query to run an insert but channel's ID wasn't populated yet so I gave up on that. I've tried using an embedded id class consisting of channel.getId and inUseAt. My equals and hashcode for are: public boolean equals(Object obj){ if(this == obj) return true; if(!(obj instanceof ChannelInUse)) return false; ChannelInUse ciu = (ChannelInUse) obj; return ( (this.inUseAt == null ? ciu.inUseAt == null : this.inUseAt.equals(ciu.inUseAt)) && (this.inUse == ciu.inUse) && (this.channel == null ? ciu.channel == null : this.channel.equals(ciu.channel)) ); } /** * hashcode generated from at, channel and inUse properties. */ public int hashCode(){ int hash = 1; hash = hash * 31 + (this.inUseAt == null ? 0 : this.inUseAt.hashCode()); hash = hash * 31 + (this.channel == null ? 0 : this.channel.hashCode()); if(inUse) hash = hash * 31 + 1; else hash = hash * 31 + 0; return hash; } } I've tried using hibernate's Entity annotation with mutable=false. I'm probably just not understanding what makes an entity unique or something. Hit the google pretty hard but can't figure this one out.

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• Is the .Net HashSet uniqueness calculation completely based on Hash Codes?

  - by RobV

  I was wondering whether the .Net HashSet<T> is based completely on hash codes or whether it uses equality as well? I have a particular class that I may potentially instantiate millions of instances of and there is a reasonable chance that some hash codes will collide at that point. I'm considering using HashSet's to store some instances of this class and am wondering if it's actually worth doing - if the uniqueness of an element is only determined on its hash code then that's of no use to me for real applications MSDN documentation seems to be rather vague on this topic - any enlightenment would be appreciated

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• How do I calculate a good hash code for a list of strings?

  - by Ian Ringrose

  Background: I have a short list of strings. The number of strings is not always the same, but are nearly always of the order of a “handful” In our database will store these strings in a 2nd normalised table These strings are never changed once they are written to the database. We wish to be able to match on these strings quickly in a query without the performance hit of doing lots of joins. So I am thinking of storing a hash code of all these strings in the main table and including it in our index, so the joins are only processed by the database when the hash code matches. So how do I get a good hashcode? I could: Xor the hash codes of all the string together Xor with multiply the result after each string (say by 31) Cat all the string together then get the hashcode Some other way So what do people think? (If you care we are using .NET and SqlServer)

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• Why is this Ruby 1.9 code resulting in an empty hash?

  - by randombits

  I'm trying to zip 3 arrays into a hash. The hash is coming up empty, though. Here's sample code to reproduce using Ruby 1.9: >> foo0 = ["a"] => ["a"] >> foo1 = ["b"] => ["b"] >> foo2 = ["c"] => ["c"] >> h = Hash[foo0.zip(foo1, foo2)] => {}

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• Perl: Why does "use strict" not let me pass a parameter hash?

  - by Thariama

  I hava a perl subroutine where i would like to pass parameters as a hash (the aim is to include a css depending on the parameter 'iconsize'). I am using the call: get_function_bar_begin('iconsize' => '32'); for the subroutine get_function_bar_begin: use strict; ... sub get_function_bar_begin { my $self = shift; my %template_params = %{ shift || {} }; return $self->render_template('global/bars /tmpl_incl_function_bar_begin.html',%template_params); } Why does this yield the error message: Error executing run mode 'start': undef error - Can't use string ("iconsize") as a HASH ref while "strict refs" in use at CheckBar.pm at line 334 Am i doing something wrong here? Is there an other way to submit my data ('iconsize') as a hash?

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• Possible to detect hash with more than one key?

  - by Sandra

  I am collecting data in a hash of hashes which looks like $VAR1 = { '502' => { 'user2' => '0' }, '501' => { 'git' => '0', 'fffff' => '755' }, '19197' => { 'user4' => '755' } }; The problem is in 501. Two keys may not occur. Is it possible to detect this? Update Fixed typo in hash.

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• How to handle #(hash) character in SEO friendly url?

  - by arvinsim

  How do you bypass the default behaviour if #(hash) which is to go to a specific part of a page? The problem that I have is that the # character is a part of the SEO friendly url which is a title and the #(hash) is part of the content (i.e. like with C#). I can't retrieve the whole string and I only get the characters before the #. Example: www.domain.com/C#-programming-book in this example I only get 'C' and not the '-programming-book' part. I am not using any javascript at the moment and would like to only use a PHP solution for this. Before anyone suggests that I used url encoding, the criteria for the seo friendly url is that it should be human readable and easily remembered. So converting the hash to '%23' does not pass the criteria. Is there no way around it?

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• Algorithmic problem - quickly finding all #'s where value %x is some given value

  - by Steve B.

  Problem I'm trying to solve, apologies in advance for the length: Given a large number of stored records, each with a unique (String) field S. I'd like to be able to find through an indexed query all records where Hash(S) % N == K for any arbitrary N, K (e.g. given a million strings, find all strings where HashCode(s) % 17 = 5. Is there some way of memoizing this so that we can quickly answer any question of this form without doing the % on every value? The motivation for this is a system of N distributed nodes, where each record has to be assigned to at least one node. The nodes are numbered 0 - (K-1) , and each node has to load up all of the records that match it's number: If we have 3 nodes Node 0 loads all records where Hash % 3 ==0 Node 1 loads all records where Hash % 3 ==1 Node 2 loads all records where Hash % 3 ==2 adding a 4th node, obviously all the assignments have to be recomputed - Node 0 loads all records where Hash % 4 ==0 ... etc I'd like to easily find these records through an indexed query without having to compute the mod individually. The best I've been able to come up with so far: If we take the prime factors of N (p1 * p2 * ... ) if N % M == I then p % M == I % p for all of N's prime factors e.g. 10 nodes : N % 10 == 6 then N % 2 = 0 == 6 %2 N % 5 = 1 == 6 %5 so storing an array of the "%" of N for the first "reasonable" number of primes for my data set should be helpful. For example in the above example we store the hash and the primes HASH PRIMES (array of %2, %3, %5, %7, ... ]) 16 [0 1 1 2 .. ] so looking for N%10 == 6 is equivalent to looking for all values where array[1]==1 and array[2] == 1. However, this breaks at the first prime larger than the highest number I'm storing in the factor table. Is there a better way?

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• How can I implement collision detection for these tiles?

  - by Fiona

  I am wondering how this would be possible, if at all. In the image below: http://i.stack.imgur.com/d8cO3.png The light brows tiles are ground, while the dark brown is background, so the player can pass over those tiles. Here's the for loops that draws the level: float scale = 1f; for (row = 0; row < currentLevel.Rows; row++) { for (column = 0; column < currentLevel.Columns; column++) { Tile tile = (Tile)currentLevel.GetTile(row, column); if (tile == null) { continue; } Texture2D texture = tile.Texture; spriteBatch.Draw(texture, new Microsoft.Xna.Framework.Rectangle( (int)(column * currentLevel.CellSize.X * scale), (int)(row * currentLevel.CellSize.Y * scale), (int)(currentLevel.CellSize.X * scale), (int)(currentLevel.CellSize.Y * scale)), Microsoft.Xna.Framework.Color.White); } } Here's what I have so far to determine where to create a Rectangle: Microsoft.Xna.Framework.Rectangle[,,,] groundBounds = new Microsoft.Xna.Framework.Rectangle[?, ?, ?, ?]; int tileSize = 20; int screenSizeInTiles = 30; var tilePositions = new System.Drawing.Point[screenSizeInTiles, screenSizeInTiles]; for (int x = 0; x < screenSizeInTiles; x++) { for (int y = 0; y < screenSizeInTiles; y++) { tilePositions[x, y] = new System.Drawing.Point(x * tileSize, y * tileSize); groundBounds[x, y, tileSize, tileSize] = new Microsoft.Xna.Framework.Rectangle(x, y, 20, 20); } } First off, I'm not sure how to initialize the array groundBounds (I don't know how big to make it). Also, I'm not entirely sure how to go about adding information to groundBounds. I want to add a Rectangle for each tile in the level. Preferably I'd only make a Rectangle for those tiles accessible by the player, and not background tiles, but that's for a different day. FYI, the map was made with a freeware program called Realm Factory.

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• vector collision on polygon in 3d space detection/testing?

  - by LRFLEW

  In the 3d fps in java I'm working on, I need a bullet to be fired and to tell if it hit someone. All visual objects in the game are defined through OpenGL, so the object it can be colliding with can be any drawable polygon (although they will most likely be triangles and rectangles anyways). The bullet is not an object, but will be treated as a vector that instantaneously moves all the way across the map (like the snipper riffle in Halo). What's the best way to detect/test collisions with the polygon and the vector. I have access to OpenCL, however I have absolutely no experience with it. I am very early in the developmental stage, so if you think there's a better way of going about this, feel free to tell me (I barley have a player model to collide with anyways, so I'm flexible with it). Thanks

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• How to find collision detection side between two objects?

  - by user2362369

  I am using box2D and I have two objects, one is bouncy ball and the other one is block. I'd like to find which side of the block is collided with, so I can only make the ball bounce when it hits the top. I tried to implement many things like fixture data and by detecting position, using manifold but not get the accurate result. I also tried to calculate distance between two object but all went wrong.

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• Only Integrating Box2D collision detection in my 2d engine?

  - by Mr.Gando

  I have integrated box2d in my engine, ( Debug Draw, etc. ) and with a world I can throw in some 2d squares/rectangles etc. I saw this post, where the user is basically not using a world for collision detection, however the user doesn't explain anything about how he's using the manifold (b2Manifold), etc. Another post, is in the cocos2d forum, ( scroll down to the user Lam in the third reply ) Could anyone help me a bit with this?, basically want to add collision detection without the need of using b2World ,etc etc. Thanks a lot!

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• Calculate angle of moving ball after collision with angled or sloped wall that is a 2D line segment

  - by Ben Mc

  If you have a "ball" inside a 2D polygon, made up of say, 4 line segments that act as bounding walls, how do you calculate the angle of the ball after the collision with the irregularly sloped wall? I know how to make the ball bounce if the wall is horizontal, vertical, or at a 45 degree angle. I also have my code setup to detect a collision with the wall. I've read about dot products and normals, but I cannot figure out how to implement these in Java / Android. I'm completely stumped and feel like I've looked up everything 10 pages deep in Google 10 times now. I'm burned out trying to figure this out, I hope someone can help.

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• Collision detection on sub views contained in 2 separate parent UIViews?

  - by Chris

  Hi All, I originally setup some conditions using CGRectIntersectsRect for some collision detection which worked fine. In the greater scale of things I only need part of the view to be detected. So originally within the ViewController it was comparing 2 UIviews. Now what I need to do is collision detection of subViews within 2 different UIViews that are contained in a view in which the view controller does the logic. My script is no longer working as I suspect CGRectIntersectsRect only compares frames within the same view? I'll keep digging to confirm this. Any ways around this? Is it possible for example to get the x and y pos of the sub view in relation to the main view that's performing the logic?

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• What does the MS Ajax Framework use location.hash for?

  - by DDaviesBrackett

  I've noticed that the MS ajax framework touches the action of the default form during Sys.Application.initialize, appending location.hash to it. This is interfering with other code in my app that expects different behaviour. What does the framework do with that? It refers to the values it puts on the hash as 'state', but how do I find out what it's communicating and from where?

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• Could someone explain hash tag usage for deeplinking ajax applications?

  - by Sergio

  Hi There, I am currently trying to full appreciate how and when to use hash tags in urls when building an ajax powered website. There seems to be a distinct lack of reading material on the web regarding this technique and as such I don't feel like I've got a good handle on it. Could someone explain in the simplest terms how the hash tag can be used in urls to enable things like loading pages via ajax. Thanks

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• Can I update window.location.hash without having the web page scroll?

  - by Jonathon Watney

  Using JavaScript, is there a way to update window.location.hash without scrolling the web page? I have clickable title elements that toggle the visibility of a div directly beneath them. I want the /foo#bar in the history when clicking titles but don't want the page scrolling about. So when navigating away from /foo#bar I'll be able to use the back button and have the div whose ID is in window.location.hash be visible upon return. Is this behavior possible?

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• How does one access this hash from an ActiveRecord class in the Rails console?

  - by randombits

  I have a simple hash I created to make it easier to do dictionary type lookups. For instance: class Numbers < ActiveRecord::Base MY_TYPES { one: => 1, two: => 2 } end Is it not possible in the rails console to access this hash via Numbers.MY_TYPES[:one]?

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• Recipient address rejected: User unknown in local recipient table;

  - by Thufir

  I've gone through the guide for mailman with some difficulty, but seem to be nearly there. I'm able to navigate to the mailman web GUI, create lists and subscribe. I just subscribe my local FQDN, so [email protected] for testing purposes. This FQDN only works on localhost. However, e-mails to the list address, in this case [email protected], are rejected: root@dur:~# root@dur:~# tail /var/log/mail.log Aug 28 08:28:43 dur postfix/master[12208]: terminating on signal 15 Aug 28 08:28:44 dur postfix/postfix-script[12322]: starting the Postfix mail system Aug 28 08:28:44 dur postfix/master[12323]: daemon started -- version 2.9.1, configuration /etc/postfix Aug 28 08:28:46 dur postfix/postfix-script[12332]: stopping the Postfix mail system Aug 28 08:28:46 dur postfix/master[12323]: terminating on signal 15 Aug 28 08:28:47 dur postfix/postfix-script[12437]: starting the Postfix mail system Aug 28 08:28:47 dur postfix/master[12438]: daemon started -- version 2.9.1, configuration /etc/postfix Aug 28 08:29:29 dur postfix/smtpd[12460]: connect from localhost[127.0.0.1] Aug 28 08:29:30 dur postfix/smtpd[12460]: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<dur.bounceme.net> Aug 28 08:29:33 dur postfix/smtpd[12460]: disconnect from localhost[127.0.0.1] root@dur:~# root@dur:~# ll /var/lib/mailman/data/ total 56 drwxrwsr-x 2 root list 4096 Aug 28 08:28 ./ drwxrwsr-x 8 root list 4096 Aug 27 19:58 ../ -rw-r--r-- 1 root list 0 Aug 28 04:36 aliases -rw-r--r-- 1 root list 12288 Aug 28 04:36 aliases.db -rw-r--r-- 1 root list 12288 Aug 28 08:28 aliases.db.db -rw-r----- 1 root list 41 Aug 27 21:04 creator.pw -rw-rw-r-- 1 root list 10 Aug 27 19:58 last_mailman_version -rw-r--r-- 1 root list 14100 Oct 19 2011 sitelist.cfg root@dur:~# root@dur:~# grep alias /etc/postfix/main.cf alias_maps = hash:/etc/aliases, hash:/var/lib/mailman/data/aliases alias_database = hash:/var/lib/mailman/data/aliases.db #alias_database = hash:/etc/aliases root@dur:~# root@dur:~# postconf -n alias_database = hash:/var/lib/mailman/data/aliases.db alias_maps = hash:/etc/aliases, hash:/var/lib/mailman/data/aliases append_dot_mydomain = no biff = no broken_sasl_auth_clients = yes config_directory = /etc/postfix default_transport = smtp home_mailbox = Maildir/ inet_interfaces = loopback-only mailbox_command = /usr/lib/dovecot/deliver -c /etc/dovecot/conf.d/01-mail-stack-delivery.conf -m "${EXTENSION}" mailbox_size_limit = 0 mailman_destination_recipient_limit = 1 mydestination = $myhostname localhost.$mydomain localhost $mydomain myhostname = dur.bounceme.net mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 readme_directory = no recipient_delimiter = + relay_domains = lists.example.com relay_transport = relay relayhost = smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache smtp_use_tls = yes smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu) smtpd_recipient_restrictions = reject_unknown_sender_domain, reject_unknown_recipient_domain, reject_unauth_pipelining, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination smtpd_sasl_auth_enable = yes smtpd_sasl_authenticated_header = yes smtpd_sasl_local_domain = $myhostname smtpd_sasl_path = private/dovecot-auth smtpd_sasl_security_options = noanonymous smtpd_sasl_type = dovecot smtpd_tls_auth_only = yes smtpd_tls_cert_file = /etc/ssl/certs/ssl-mail.pem smtpd_tls_key_file = /etc/ssl/private/ssl-mail.key smtpd_tls_mandatory_ciphers = medium smtpd_tls_mandatory_protocols = SSLv3, TLSv1 smtpd_tls_received_header = yes smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache smtpd_use_tls = yes tls_random_source = dev:/dev/urandom transport_maps = hash:/etc/postfix/transport root@dur:~# Why is this e-mail rejected? It seems to, maybe be related to the alias_maps and alias_database settings in postfix.

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• /etc/postfix/transport missing; what should it look like?

  - by Thufir

  I'm following the mailman guide but couldn't locate /etc/postfix/ so created it as follows: root@dur:~# root@dur:~# cat /etc/postfix/transport dur.bounceme.net mailman: root@dur:~# root@dur:~# telnet localhost 25 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. 220 dur.bounceme.net ESMTP Postfix (Ubuntu) ehlo fqdn_test 250-dur.bounceme.net 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-STARTTLS 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN mail from:[email protected] 250 2.1.0 Ok rcpt to:thufir@localhost 451 4.3.0 <thufir@localhost>: Temporary lookup failure rcpt to:[email protected] 451 4.3.0 <[email protected]>: Temporary lookup failure quit 221 2.0.0 Bye Connection closed by foreign host. root@dur:~# root@dur:~# postconf -n alias_database = hash:/etc/aliases alias_maps = hash:/etc/aliases, hash:/var/lib/mailman/data/aliases append_dot_mydomain = no biff = no broken_sasl_auth_clients = yes config_directory = /etc/postfix default_transport = smtp home_mailbox = Maildir/ inet_interfaces = loopback-only mailbox_command = /usr/lib/dovecot/deliver -c /etc/dovecot/conf.d/01-mail-stack-delivery.conf -m "${EXTENSION}" mailbox_size_limit = 0 mailman_destination_recipient_limit = 1 mydestination = dur, dur.bounceme.net, localhost.bounceme.net, localhost myhostname = dur.bounceme.net mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 readme_directory = no recipient_delimiter = + relay_domains = lists.dur.bounceme.net relay_transport = relay relayhost = smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache smtp_use_tls = yes smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu) smtpd_recipient_restrictions = reject_unknown_sender_domain, reject_unknown_recipient_domain, reject_unauth_pipelining, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination smtpd_sasl_auth_enable = yes smtpd_sasl_authenticated_header = yes smtpd_sasl_local_domain = $myhostname smtpd_sasl_path = private/dovecot-auth smtpd_sasl_security_options = noanonymous smtpd_sasl_type = dovecot smtpd_tls_auth_only = yes smtpd_tls_cert_file = /etc/ssl/certs/ssl-mail.pem smtpd_tls_key_file = /etc/ssl/private/ssl-mail.key smtpd_tls_mandatory_ciphers = medium smtpd_tls_mandatory_protocols = SSLv3, TLSv1 smtpd_tls_received_header = yes smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache smtpd_use_tls = yes tls_random_source = dev:/dev/urandom transport_maps = hash:/etc/postfix/transport root@dur:~# root@dur:~# tail /var/log/mail.log Aug 28 02:05:15 dur postfix/smtpd[20326]: connect from localhost[127.0.0.1] Aug 28 02:06:10 dur postfix/smtpd[20326]: warning: hash:/var/lib/mailman/data/aliases is unavailable. open database /var/lib/mailman/data/aliases.db: No such file or directory Aug 28 02:06:10 dur postfix/smtpd[20326]: warning: hash:/var/lib/mailman/data/aliases lookup error for "thufir@localhost" Aug 28 02:06:10 dur postfix/smtpd[20326]: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 451 4.3.0 <thufir@localhost>: Temporary lookup failure; from=<[email protected]> to=<thufir@localhost> proto=ESMTP helo=<fqdn_test> Aug 28 02:06:23 dur postfix/smtpd[20326]: warning: hash:/var/lib/mailman/data/aliases is unavailable. open database /var/lib/mailman/data/aliases.db: No such file or directory Aug 28 02:06:23 dur postfix/smtpd[20326]: warning: hash:/var/lib/mailman/data/aliases lookup error for "[email protected]" Aug 28 02:06:23 dur postfix/smtpd[20326]: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 451 4.3.0 <[email protected]>: Temporary lookup failure; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<fqdn_test> Aug 28 02:06:28 dur postfix/smtpd[20326]: disconnect from localhost[127.0.0.1] Aug 28 02:06:49 dur dovecot: pop3-login: Login: user=<thufir>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20338, TLS Aug 28 02:06:49 dur dovecot: pop3(thufir): Disconnected: Logged out top=0/0, retr=0/0, del=0/0, size=0 root@dur:~# The manual page is here.

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• Optimizing multiple dispatch notification algorithm in C#?

  - by Robert Fraser

  Sorry about the title, I couldn't think of a better way to describe the problem. Basically, I'm trying to implement a collision system in a game. I want to be able to register a "collision handler" that handles any collision of two objects (given in either order) that can be cast to particular types. So if Player : Ship : Entity and Laser : Particle : Entity, and handlers for (Ship, Particle) and (Laser, Entity) are registered than for a collision of (Laser, Player), both handlers should be notified, with the arguments in the correct order, and a collision of (Laser, Laser) should notify only the second handler. A code snippet says a thousand words, so here's what I'm doing right now (naieve method): public IObservable<Collision<T1, T2>> onCollisionsOf<T1, T2>() where T1 : Entity where T2 : Entity { Type t1 = typeof(T1); Type t2 = typeof(T2); Subject<Collision<T1, T2>> obs = new Subject<Collision<T1, T2>>(); _onCollisionInternal += delegate(Entity obj1, Entity obj2) { if (t1.IsAssignableFrom(obj1.GetType()) && t2.IsAssignableFrom(obj2.GetType())) obs.OnNext(new Collision<T1, T2>((T1) obj1, (T2) obj2)); else if (t1.IsAssignableFrom(obj2.GetType()) && t2.IsAssignableFrom(obj1.GetType())) obs.OnNext(new Collision<T1, T2>((T1) obj2, (T2) obj1)); }; return obs; } However, this method is quite slow (measurable; I lost ~2 FPS after implementing this), so I'm looking for a way to shave a couple cycles/allocation off this. I thought about (as in, spent an hour implementing then slammed my head against a wall for being such an idiot) a method that put the types in an order based on their hash code, then put them into a dictionary, with each entry being a linked list of handlers for pairs of that type with a boolean indication whether the handler wanted the order of arguments reversed. Unfortunately, this doesn't work for derived types, since if a derived type is passed in, it won't notify a subscriber for the base type. Can anyone think of a way better than checking every type pair (twice) to see if it matches? Thanks, Robert

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• What is the best way to solve an Objective-C namespace collision?

  - by Mecki

  Objective-C has no namespaces; it's much like C, everything is within one global namespace. Common practice is to prefix classes with initials, e.g. if you are working at IBM, you could prefix them with "IBM"; if you work for Microsoft, you could use "MS"; and so on. Sometimes the initials refer to the project, e.g. Adium prefixes classes with "AI" (as there is no company behind it of that you could take the initials). Apple prefixes classes with NS and says this prefix is reserved for Apple only. So far so well. But appending 2 to 4 letters to a class name in front is a very, very limited namespace. E.g. MS or AI could have an entirely different meanings (AI could be Artificial Intelligence for example) and some other developer might decide to use them and create an equally named class. Bang, namespace collision. Okay, if this is a collision between one of your own classes and one of an external framework you are using, you can easily change the naming of your class, no big deal. But what if you use two external frameworks, both frameworks that you don't have the source to and that you can't change? Your application links with both of them and you get name conflicts. How would you go about solving these? What is the best way to work around them in such a way that you can still use both classes? In C you can work around these by not linking directly to the library, instead you load the library at runtime, using dlopen(), then find the symbol you are looking for using dlsym() and assign it to a global symbol (that you can name any way you like) and then access it through this global symbol. E.g. if you have a conflict because some C library has a function named open(), you could define a variable named myOpen and have it point to the open() function of the library, thus when you want to use the system open(), you just use open() and when you want to use the other one, you access it via the myOpen identifier. Is something similar possible in Objective-C and if not, is there any other clever, tricky solution you can use resolve namespace conflicts? Any ideas? Update: Just to clarify this: answers that suggest how to avoid namespace collisions in advance or how to create a better namespace are certainly welcome; however, I will not accept them as the answer since they don't solve my problem. I have two libraries and their class names collide. I can't change them; I don't have the source of either one. The collision is already there and tips on how it could have been avoided in advance won't help anymore. I can forward them to the developers of these frameworks and hope they choose a better namespace in the future, but for the time being I'm searching a solution to work with the frameworks right now within a single application. Any solutions to make this possible?

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• How does the rsync algorithm correctly identify repeating blocks?

  - by Kai

  I'm on a personal quest to learn how the rsync algorithm works. After some reading and thinking, I've come up with a situation where I think the algorithm fails. I'm trying to figure out how this is resolved in an actual implementation. Consider this example, where A is the receiver and B is the sender. A = abcde1234512345fghij B = abcde12345fghij As you can see, the only change is that 12345 has been removed. Now, to make this example interesting, let's choose a block size of 5 bytes (chars). Hashing the values on the sender's side using the weak checksum gives the following values list. abcde|12345|fghij abcde -> 495 12345 -> 255 fghij -> 520 values = [495, 255, 520] Next we check to see if any hash values differ in A. If there's a matching block we can skip to the end of that block for the next check. If there's a non-matching block then we've found a difference. I'll step through this process. Hash the first block. Does this hash exist in the values list? abcde -> 495 (yes, so skip) Hash the second block. Does this hash exist in the values list? 12345 -> 255 (yes, so skip) Hash the third block. Does this hash exist in the values list? 12345 -> 255 (yes, so skip) Hash the fourth block. Does this hash exist in the values list? fghij -> 520 (yes, so skip) No more data, we're done. Since every hash was found in the values list, we conclude that A and B are the same. Which, in my humble opinion, isn't true. It seems to me this will happen whenever there is more than one block that share the same hash. What am I missing?

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• Working with a large data object between ruby processes

  - by Gdeglin

  I have a Ruby hash that reaches approximately 10 megabytes if written to a file using Marshal.dump. After gzip compression it is approximately 500 kilobytes. Iterating through and altering this hash is very fast in ruby (fractions of a millisecond). Even copying it is extremely fast. The problem is that I need to share the data in this hash between Ruby on Rails processes. In order to do this using the Rails cache (file_store or memcached) I need to Marshal.dump the file first, however this incurs a 1000 millisecond delay when serializing the file and a 400 millisecond delay when serializing it. Ideally I would want to be able to save and load this hash from each process in under 100 milliseconds. One idea is to spawn a new Ruby process to hold this hash that provides an API to the other processes to modify or process the data within it, but I want to avoid doing this unless I'm certain that there are no other ways to share this object quickly. Is there a way I can more directly share this hash between processes without needing to serialize or deserialize it? Here is the code I'm using to generate a hash similar to the one I'm working with: @a = [] 0.upto(500) do |r| @a[r] = [] 0.upto(10_000) do |c| if rand(10) == 0 @a[r][c] = 1 # 10% chance of being 1 else @a[r][c] = 0 end end end @c = Marshal.dump(@a) # 1000 milliseconds Marshal.load(@c) # 400 milliseconds Update: Since my original question did not receive many responses, I'm assuming there's no solution as easy as I would have hoped. Presently I'm considering two options: Create a Sinatra application to store this hash with an API to modify/access it. Create a C application to do the same as #1, but a lot faster. The scope of my problem has increased such that the hash may be larger than my original example. So #2 may be necessary. But I have no idea where to start in terms of writing a C application that exposes an appropriate API. A good walkthrough through how best to implement #1 or #2 may receive best answer credit.

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• WPF data-bound ComboBox only shows first item of ItemsSource list

  - by Mark

  Hi all, I'm sure I'm doing something stupid but for the life of me I can't think of right now. I have a ComboBox that is data-bound to a list of Layout objects. The list is initially empty but things are added over time. When the list is updated by the model the first time, this update reflects properly in the ComboBox. However, subsequent updates never show up in the ComboBox even though I can see that the list itself contains these items. Since the first update works, I know the data-binding is OK - so what am I doing wrong here? Here's the XAML (abridged): <Grid HorizontalAlignment="Stretch"> <ComboBox ItemsSource="{Binding Path=SavedLayouts, diagnostics:PresentationTraceSources.TraceLevel=High}" DisplayMemberPath="Name" SelectedValuePath="Name" SelectedItem="{Binding LoadLayout}" Height="25" Grid.Row="1" Grid.Column="0"></ComboBox> </Grid> And the related part of the model: public IList<Layout> SavedLayouts { get { return _layouts; } } public Layout SaveLayout( String data_ ) { Layout theLayout = new Layout( SaveLayoutName ); _layouts.Add( theLayout ); try { return theLayout; } finally { PropertyChangedEventHandler handler = PropertyChanged; if( handler != null ) { handler( this, new PropertyChangedEventArgs( "SavedLayouts" ) ); } } } And finally, the layout class (abridged): public class Layout { public String Name { get; private set; } } In the output window, I can see the update occurring: System.Windows.Data Warning: 91 : BindingExpression (hash=64564967): Got PropertyChanged event from TickerzModel (hash=43624632) System.Windows.Data Warning: 97 : BindingExpression (hash=64564967): GetValue at level 0 from TickerzModel (hash=43624632) using RuntimePropertyInfo(SavedLayouts): List`1 (hash=16951421 Count=11) System.Windows.Data Warning: 76 : BindingExpression (hash=64564967): TransferValue - got raw value List`1 (hash=16951421 Count=11) System.Windows.Data Warning: 85 : BindingExpression (hash=64564967): TransferValue - using final value List`1 (hash=16951421 Count=11) But I get do not get this 11th item in the ComboBox. Any ideas?

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